This invention relates generally to a test method for determining residual stress in non-ferromagnetic metal objects and more particularly to a novel eddy current measurement procedure enabling extremely minor conductivity changes in such metal objects to now be reliably measured.
As a non-destructive test procedure, eddy current measurement has long been employed for detection of relatively macroscopic defects or compositional variations in non-ferromagnetic metal objects such as surface and sub-surface cracks or flaws, irregularities in material structure and still other variations in metallurgical characteristics. Various commercially available testing equipment is employed for such measurements to include automatically controlled measurement means such as the smart Eddy.TM. system, manufactured by FaAA Products Corporation, Menlo Park, Calif., which utilizes software programmed general purpose computer means operatively associated with an alternating current bridge measuring circuit. The commercially available test equipment does not reliably detect much smaller conductivity changes resulting from residual stress in these metal objects, however, thereby requiring still other means for measurement of these important physical characteristics. To further explain, residual stress in titanium and aluminum alloys such as near surface compression or tension resulting from mechanical action such as shot-peening or rolling has not thus far been reliably detected with the commercially available test equipment. Thus, a customary displacement or lift-off measurement procedure employed with the commercially available test equipment does not produce accurate impedance level changes indicative of the residual stress condition existing in the metal object being tested. It remains desirable, therefore, to find an alternate test procedure whereby extremely small impedance level differences attributable to the existing stress condition in the tested metal object is reliably detected.
Accordingly, it is an important object of the present invention, therefore, to provide an extremely sensitive eddy current test procedure for measurement of residual stress in non-ferromagnetic metal objects.
It is another important object of the present invention to modify conventional eddy current test equipment in a manner enabling accurate detection of the existing residual stress condition in non-ferromagnetic metal objects.
It is a still further important object of the present invention to significantly increase the detection sensitivity of conventional eddy current test equipment in a simplified manner not requiring extensive modification of its component parts.
These and further important objects of the present invention will become more apparent upon considering the following detailed description of the present invention.